Translation:Tomorrow dad will understand why we did that.
In certain cases, including this one, "fari" can be translated as "make", so you should report if Duolingo said wrong to your answer. In the section numbered "I" of the entry for "fari" on the Plena Ilustrita Vortaro de Esperanto, all the examples are sentences or expressions for which the verb "fari" should be translated as "make" (for example, "abeloj faras ĉelojn el vakso" translates as "bees make cells from wax"). http://vortaro.net/#fari
I could be convinced that a contrived context exists where someone would say "Kial vi faris tion" when they want to ask "why did you make that -- but it seems to me that this contrived context is so excedingly rare that it can be safely discounted. (People misspell their own names more often!)
I spent a little bit of time on my own challenge and failed -- that is, I couldn't find any cases where "faris tion" had to do with having made something. Perhaps it would be helpful to think of "fari" as "to do" and sometimes "to make" if the context requires it.